The present invention relates to rotary fluid pressure devices, and more particularly, to such devices which include an internal gear set and a pair of relatively movable valve members operable to communicate fluid to and from the gear set.
Although it should become apparent from the subsequent description that the invention may be useful with various types and configurations of rotary fluid pressure devices, including both pumps and motors, it is especially advantageous when used in a device including a gerotor gear set.
Fluid motors of the type utilizing a gerotor displacement mechanism to convert fluid pressure into a rotary output are especially suited for low-speed, high-torque applications. Typically, in fluid motors of this type, there are two relatively movable valve members, one of which is stationary and provides a fluid passage communicating with each of the volume chambers of the gerotor, while the other valve member rotates at the speed of rotation of the rotatable member of the gerotor gear set. Valving of the type described above is referred to as being "low-speed, commutating" valving, to distinguish it from the type of valving referred to as "high-speed" valving, wherein the rotatable valve member rotates at the orbit speed of the orbiting member of the gerotor set.
One of the important performance criteria in gerotor motors of the type having low-speed, commutating valving is the "no-load pressure drop", which is a measure of the mechanical efficiency of the motor. The no-load pressure drop is the difference between the pressure at the inlet port and the pressure at the outlet port which is required to rotate the output shaft of the motor, with "no load", or no resistance to rotation of the output shaft. In a sense, the no-load pressure drop may be considered a measure of the motor's resistance to fluid flow through the main flow path, from the inlet port through the valving, then through the gerotor, then back through the valving, and finally to the outlet port. The smaller the various fluid passages and ports, the greater the resistance or restriction to fluid flow, and the higher the no-load pressure drop.
Excessive no-load pressure drop has been a problem especially in gerotor motors of the type referred to as "disc valve" motors, such as is shown in U.S. Pat. Nos. 3,572,983 and 3,434,600, assigned to the assignee of the present invention and incorporated herein by reference. The term "disc valve" will be understood to refer to a device in which the stationary and rotary valve surfaces are both, flat, planar surfaces oriented transverse to the axis of rotation of the device. In disc valve motors, there is a rotary disc valve defining a plurality of valve ports (for example, 12 or more) in a relatively small area, thus limiting the size of the ports and the area of communication between the rotating ports and the adjacent stationary ports.
Related to the problem of no-load pressure drop is that typical disc valve gerotor motors have not commonly been used as pumps, because the relatively high restriction to fluid flow would cause cavitation to occur within the device, unless the inlet port is pressurized, such as by means of a charge pump.